Like the PP said, you can make a PS for any DLR restaurant up to 60 days in advance. My thought it why wait to do it though? It's best to do it as early as possible to ensure that the date/time you want is available. Disney is very accommodating and don't mind if you need to make changes later. GK is pretty popular and can get very crowded during peak season/holidays so don't wait too long to make that call!